The function of the data transmission element is to sense the controlled output quantity and to convert it to a signal which can be used to either monitor the output or to act as a feedback device in a closed loop control system. The controlled output variable in a hydraulically operated force-motion control system can be:
(1) Force.
(2) Velocity.
(3) Position.
(4) Acceleration/deceleration.
(5) Pressure.
(6) Flow.
Although the output signal produced by the data transmission element can take many forms, the most common type used in modern control applications will be a transducer creating an electrical signal, usually at a low power level. Transducers measuring any of the six listed output variables are commercially available.
1 Force
Force transducers are termed load cells. They directly sense the load exerted by the actuator or a load-bearing member of the system. Most commercially available load cells are designed to measure loading acting in one direction only.
2 Velocity
Velocity transducers are usually referred to as tachos. the most readily available having a rotary input which is converted into an AC or DC
25
26 HYDRAULIC AND ELECTRO-HYDRAULIC CONTROL SYSTEMS
output voltage directly proportional to input spindle velocity. DC tachos are more suitable for closed loop control applications. Tachos can have a wide operating range but have a threshold which limits the minimum running speed. In applications where linear motion has to be sensed, some form of rack and pinion gearing or cable drive has to be employed, and care has to be taken that backlash is eliminated, since the presence of backlash can have an adverse effect on system stability and performance.
3. Position
Position transducers can have a linear or rotary input and either an AC or DC output signal. The output signal can be a voltage or current, usually at low power level. DC position transducers, or potentiometers, are variable resistances of wire-wound or plastic film construction. The resolution of wire-wound potentiometers is limited by the pitch of the windings, whereas plastic film potentiometers have infinite resolution. The resistance value of the potentiometer has a bearing on the linearity of the input-output characteristics; 1 kfl is a fairly typical value. Since potentiometers are contacting devices, their life and reliability are limited, particularly if they are used as feedback sensors. It is common practice to use potentiometers as reference or demand signal generators and non-contacting devices as feedback sensors.
Non-contacting transducers can produce either an analogue or a digital output signal, analogue devices providing an infintely variable output related to an absolute datum, digital devices providing a discrete incremental output without an absolute datum. Typical examples of non-contacting analogue transducers are LVDTs, RVDTs, synchros and resolvers; examples of digital transducers are encoders and proximity sensors.
Inductive devices producing an AC output signal are termed LVDTs, i.e.
linear variable displacement transducers or RVDTs, i.e. rotary variable displacement transducers. Some typical circuits are shown in Fig. 6.1.
Packaged units incorporating integrated oscillator-demodulator circuitry, thus providing a DC output, are readily available.
Synchros and resolvers are rotary AC transducers generating a sinusoidal ouput. An example of a circuit employing two synchros, one acting as a demand transducer, the other as a feedback element, is shown in Fig. 6.2.
Non-contacting transducers have a virtually unlimited life and infinite resolution, but their linearity is usually inferior to that of potentiometers.
INDUCTIVE PICK·OFF r---.., EXCI~ L---...i ( 0) CIRCUIT DIAGRAM EMPLOYING 2 PICK-OFFS SERIES AOO!TION
INDUCTIVE PICK-OFF r----, E)(;;ITATIOI!i
j--, OOTATdl
l, ___ ...J (b) CIRCUIT DIAGRAM EMPLOYING 2 PICK·OfFS STAR ADDITION ROTARY PICK-OFFL ___ ..J
INPUT SIGNAL
+
FEEDBACK SIGNAL-~~:· "''E
~Ji~
. -
'---" ~ (c) CIRCUIT DIAGRAM EMPLOYING INDUCTIVE PICK·OFF AND POTENTIOMETER ELECTRO HYDRAULIC . SERVO VALVEj
L---+--TO
TANK HYDRAULIC SUPPLY Fig. 6.1 Positional control system using LVDT/RVDT.
0 > --l > ~ >
z
~ ~~
m f;; ~~
N -...j28 HYDRAULIC AND ELECTRO-HYDRAULIC CONTROL SYSTEMS
OUTPUT SYNCHf\0 INPUT SYNCHRO
OUTPUT
TO~---=~ t~=----~
EXCITATION... :::.:_.0~_1~---trd_ __ ?.:l'"'" '"'"'"'
Fig. 6.2 Positional control system using synchros.
r l
Fig. 6.3 Bidirectional flow transducer.
DATA TRANSMISSION ELEMENTS 29
4. Acceleration
Although acceleration sensors are on the market, they are not widely employed in hydraulic control systems. Since acceleration is the first time derivative of velocity and the second time derivative of displacement, both acceleration and deceleration are easily handled in motion control systems by controlling the rate of change of velocity. Various methods of accomplishing this will be discussed in subsequent chapters.
5. Pressure
Pressure transducers fall into two distinct categories, i.e. single and differential pressure versions. In a single pressure transducer, pressure is sensed by means of a diaphragm, bellows or spring arrangement and the resulting displacement converted to an electrical signal by using a position transducer, giving a linear relationship between pressure and output signal.
Differential pressure transducers measure the pressure difference at two different pressure tappings and are required to provide a signal proportional to pressure differential unaffected by base pressure variations.
Construction of differential pressure transducers is similar to that of single pressure units.
6. Flow
Three types of flow transducers can be used: positive displacement units, piezo-electric venturi meters, and variable resistance flow meters. The most suitable for hydraulic control applications are positive displacement units and variable resistance flow meters. The former are essentially hydraulic motors using either tachos or rotary position transducers, while the latter make use of the principle of hydro-kinetic flow to convert flow to pressure which in turn is converted into a corresponding displacement and hence an electrical output signal. An example of such a device is shown in Fig. 6.3.
Another function of the data transmission elements is to generate and process the input signal. In this sense they constitute the element generically described as the controller. Various types of controllers will be discussed in the following chapter.